Abundant supplies of fresh water are essential to the development of industry. Enormous quantities are required for the cooling of products and equipment, for process needs, for boiler feed, and for sanitary and portable water supply. It is becoming increasingly apparent that fresh water is a valuable resource that must be protected through proper management, conservation, and use. In order to ensure an adequate supply of high quality water for industrial use, the following practices must be implemented: (1) purification and conditioning prior to consumer (portable) or industrial use; (2) conservation (and reuse where possible); (3) wastewater treatment.
The solvency power of water can pose a major threat to industrial equipment. Corrosion reactions cause the slow dissolution of metals by water and eventually structural failure of process equipment. Deposition reactions, which produce scale on heat transfer surfaces and cause loss of production, represent a change in the solvency power of water as its temperature is varied. The control of corrosion and scale is a major focus of water treatment technology.
Typical industrial water systems are subject to considerable variation. The characteristics of water composition can change over time. The abruptness and degree of change depend upon the source of the water. Water losses from a recirculating system, changes in production rates, and chemical feed rates all introduce variation into the system and thereby influence the ability to maintain proper control of the system.
Typically, given a particular calcium ion content in water, a treatment comprised of an inorganic orthophosphate together with a water soluble polymer is used to form a protective film on metallic surfaces in contact with aqueous systems, in particular cooling water systems, to thereby protect such from corrosion. The water soluble polymer is critically important to control calcium phosphate crystallization so that relatively high levels of orthophosphate may be maintained in the system to achieve the desired protection without resulting in fouling or impeded heat transfer functions which normally be caused by calcium phosphate deposition. Water soluble polymers are also used to control the formation of calcium sulfate and calcium carbonate and additionally to dispense particulates to protect the overall efficiency of water systems.
U.S. Pat. No. 5,171,450, established a simplified recognition that the phenomenon of scaling or corrosion in cooling towers can be inhibited by selection of an appropriate polymer, or combination of polymers, as the treating agent. This was based on the fact that losses of the active polymer as a consequence of attrition due to protective film formation on equipment or avoiding deposits by adsorbing onto solid impurities to prevent agglomeration or crystal growth of particulates which can deposit on the equipment. In this patent, the active polymer is defined as the polymer measured by its fluorescent tags, and active polymer loss is defined by using an inert chemical tracer (measure of total product concentration) and subtracting active polymer concentration as indicated from tagged polymer level. Thus, the control of corrosion and scaling is accomplished by control of active polymer at a level where active component losses are not excessive.
In U.S. Pat. No. 6,153,110, polymer inhibition efficiency was defined, i.e. the ratio of free polymer level to total polymer level. In defining free and total polymer levels, the polymer lost from the system undetected by sampling the system water was excluded initially, then free polymer was defined as unreacted polymer, and bounded polymer was defined as both polymer associated with inhibited particles (functioning as a scale inhibitor) and polymer absorbed onto undeposited scale (functioning as a dispersant). The free and bounded polymer together comprised the total polymer present in the water system. A correlation was established between % polymer inhibition efficiency and % scale inhibition, and between % polymer inhibition efficiency and % particulate dispersion. Thus, the control of scaling and deposition was accomplished by controlling at the required ratio of free polymer level to total polymer level.
U.S. Pat. No. 5,171,450 and U.S. Pat. No. 6,153,110 took an over simplified viewpoint of the problems to be addressed. In reality, the primary factors for scaling control are pH, hardness and temperature, while the polymer is the secondary factor. See for example Table I below, showing different active polymer concentrations required at different pH levels. By not controlling the primary effect of pH on scaling, in case of increasing pH, control wind up results in an uneconomical consumption of polymer.
TABLE IRequired activepHpolymer conc., ppm7.227.447.677.812
Additionally, the present inventors have noted that the controlled variables in U.S. Pat. No. 5,171,450 and U.S. Pat. No. 6,153,110 have no direct linkage to site specific key performance parameters such as corrosion and scaling. Every industrial water system is unique. In operating systems, proper treatment often requires constant adjustment of the chemistry to meet the requirements of rapidly changing system conditions. What is a suitable target of polymer loss or % polymer inhibition efficiency for one system at one time may not be suitable for the same system at another time or for another system. Without direct measurement of performance, polymer concentration monitoring provides no assurance for site specific performance.
A third issue with the processes currently available is that monitoring of polymer concentration, and its derivatives such as polymer loss and % polymer inhibition efficiency, cannot detect localized scaling at hot surfaces, which is only correlated with the absolute amount of polymer loss over specific surfaces. The smaller the surface, the larger the system volume, the less likely the absolute amount of polymer loss due to localized scaling is reflected in polymer concentration changes. For example, in a 30-liter lab testing unit, a hot tube of 0.5 in diameter and 5 in length is heavily scaled. Yet, the absolute polymer loss at the surface divided by the system volume is not reflected in polymer concentration change. For instance, if the same surface to volume ratio applies to a real world cooling tower of 450,000 gal, then the absolute amount of polymer loss due to scaling at 3000 ft2 heat transfer surface is un-observable from polymer concentration changes.
A fourth concern is that, a feedback control of polymer level based on polymer loss and % polymer inhibition efficiency would likely result in an uneconomical consumption of polymer. Adding polymer does not help reduce the absolute amount of bounded polymer already existing in the system, simply because polymer that bounds to undeposited scale will not be released from scale. The reduction of the absolute amount of bounded polymer only depends on system blowdown rate. To achieve the same % polymer inhibition efficiency requires a higher polymer level when bounded polymer is not fully depleted from the system. For instance, to achieve the same 90% polymer inhibition efficiency requires 10 ppm total polymer with 1 ppm bounded polymer, but 20 ppm total polymer with 2 ppm bounded polymer.
U.S. Pat. No. 6,510,368 and U.S. Pat. No. 6,068,012 proposed performance based control systems by directly measuring performance parameters such as corrosion, scaling and fouling, for the obvious reasons that monitoring inert chemical tracer leads to control wind down of active chemical, and monitoring active chemical leads to control wind up of total chemical feed, and neither provides assurance for site specific performance. In both patents, a decision tree was developed to diagnose from performance measurements the causes of performance degradation and take corrective actions accordingly.
A key disadvantage of the above performance based control systems is that they are reactive instead of proactive, in other words, corrosion, scaling and fouling are already actively occurring in the system. Moreover, corrosion, scaling and fouling are highly inter-correlated. Once commenced, one will trigger and intensify the other two, which may demand three or four times of more chemicals to bring the system back to its performance baseline, thus resulting in an uneconomical consumption of chemicals. Maintaining the health of an industrial water system proactively is more economical than trying to fix an unhealthy one. Therefore, what is still needed within the industry is a control system that is proactive instead or reactive, and therefore results in more efficient and economical processes.